Integrable conductivity measuring device

ABSTRACT

A conductivity measuring device has a current source device connected to   current supply elements through which a substantially square-wave current can be fed into a liquid, and a measuring circuit for determining the voltage drop in the liquid between voltage measuring elements. In order to provide such an integrable conductivity measuring device which prevents polarization effects, which avoids a galvanic connection to the liquid whose conductivity is to be measured, and which permits a simple detection of the conductivity of said liquid, it is suggested that the measuring circuit should be a switch-capacitor circuit including a measuring capacitor, a differential amplifier having a feedback capacitor arranged in its feedback branch, and a switch by means of which the two connections of the measuring capacitor are connected in time dependence on the behavior of the square-wave current to the voltage electrodes in one switching state and to the two inputs of the differential amplifier in another switching state, and that the current supply elements should be implemented as current coupling capacitors.

RELATED PATENT APPLICATION

Major aspects of the subject matter of this application are related tothe subject matter of U.S. Ser. No. 08/119,243 pending corresponding toPCT/DE92/00242.

DESCRIPTION

The present invention refers to an integrable conductivity measuringdevice for measuring the electric conductivity of liquids.

For the purpose of determining the electric conductivity of a liquid, itis generally known to impress a current upon said liquid and to measurethe voltage drop within said liquid, said voltage drop being inverselyproportional to the conductivity of the liquid.

In the simplest case, only two electrodes are used for this purpose. Thecurrent is impressed upon the liquid via these two electrodes and,simultaneously, the voltage drop is measured by means of the sameelectrodes. On this occasion, so-called polarization effects occur,which distort the actual measuring signal. These effects occur whenevera current flows over a boundary layer between an electrode and anelectrolyte. In view of the act that a flow of current in an electrolyteentails ion migration, ions of one type of charge will accumulate at theboundary layer between the electrolyte and the electrode, said ionaccumulations weakening the original field and reducing the measuringsignal.

In order to avoid this disadvantage, conductivity measuring devices witha socalled four-electrode arrangement are used, wherein a current sourcewith two current electrodes is provided for impressing a measuringcurrent. Two further electrodes, which can be referred to as voltageelectrodes, serve to measure the voltage dropping across the liquid. Thevoltage drop, which is tapped by the voltage electrodes, is amplified bya high-ohmic amplifier connected downstream of said voltage electrodes.On account of the high input impedance of the amplifiers, the currentflowing via the voltage electrodes can be kept small so that thepolarization effects will be reduced when this measuring method is used,and this will result in an improved measuring accuracy in comparisonwith conductivity measurements making use of only two electrodes.However, also in the four-electrode arrangement the measuring currentflowing via the voltage electrodes will cause a polarization and,consequently, a distortion of the measuring signal.

U.S. Pat. No. 4,656,427 already discloses a conductivity measuringdevice of the type mentioned at the beginning, which works with afour-electrode arrangement. Two current electrodes are adapted to beacted upon by a square-wave current. Two voltage electrodes, which arefollowed by capacitors for separating dc potentials, serve to detect thedynamic change in potential caused by the square-wave alternatingcurrent impressed upon the liquid to be measured. This signal isamplified by a subsequent evaluation circuit, which is provided with anadditional capacitor connected in series and used for separating offsetcurrents, and, after an analog-to-digital conversion, it is displayed ona display unit.

In conductivity measuring devices of the type described hereinbefore, itis also known to produce the impressed current as a sinusoidalalternating current so as to prevent by means of this measuredecomposition processes in the liquid, which would occur if themeasurement were carried out with an impressed direct current.

In order to eliminate these problems of the known integrableconductivity measuring devices comprising two voltage electrodes and twocurrent electrodes, the prior, not-prepublished PCT/DE92/00242 (WO92/18856) corresponding to U.S. Ser. No. 08/119,243 suggests that thecurrent source device should produce a square-wave current which issupplied to the two current electrodes, and that the measuring circuitshould be implemented as a switch-capacitor circuit provided with ameasuring capacitor which is adapted to be coupled to and separated fromthe voltage electrodes via a switch means in time-dependence on thebehaviour of the substantially square-wave current.

The measuring errors occurring due to polarization effects in theprepublished prior art are completely avoided with the aid of thesubject matter of PCT/DE92/00242 (WO 92/18856).

The impression of the current upon the liquid (the electrolyte) as wellas the measurement of the voltage are carried out via a direct galvaniccontact between the conductivity measuring device and the electrolyte.For this purpose, electrodes are used, which consist of precious metals,steel or carbon and which are referred to as socalled "Kohlrausch"cells. The galvanic contact results, however, in undesiredelectrochemical effects at the boundary layer between the electrodes andthe electrolyte.

These effects are, among others, the following ones:

Additional voltage drops, primarily when a current flows, which have tobe compensated for in the measurement procedure.

Electrolysis processes, i.e. discharges and depositions of ions on theelectrodes.

Absorption of ions and contamination of the electrodes, which may causedrift errors.

In the case of malfunction, a direct-current path through theelectrolyte exists, and this direct-current path may cause anelectrolysis of the electrolyte. In specific cases of use, e.g. in thefield of invasive medical diagnostics, this will have to be compensatedfor by additional measures in the field of circuit technology.

Due to the galvanic coupling, the electrolyte is at a fixed potential,and this is undesirable in connection with specific cases of use.

The presence of a metal or of some other conductive material may causeundesirable chemical reactions, one of these reactions being, forexample, the catalytic effect of platinum.

In order to avoid the above-mentioned effects, contactless measurementmethods for determining the conductivity of liquids have already beendeveloped. In connection with these measurement methods, a distinctionhas to be made between two principles: one of said methods works withinductive coupling between a measuring circuit and the electrolyte,whereas the other method uses capacitive coupling.

The inductive method comprises e.g. the feature that the electrolyte,which has been introduced in a closed tube, effects coupling ofotherwise separated windings of a transformer. A primary winding hasapplied thereto an alternating voltage, which will cause a flow ofcurrent in the electrolyte. This flow of current generates in asecondary winding a voltage whose value depends on the conductivity ofthe electrolyte. The expenditure for the apparatuses required for thismethod is, however, very high.

A capacitive coupling in accordance with the other one of theabove-mentioned measurement methods is achieved by constructing part ofthe walls of the vessel containing the electrolyte as capacitors. Partof such a capacitor is formed by a metallic layer which is covered by athin layer of glass. The respective counterelectrode is formed by theelectrolyte. The electric equivalent circuit diagram of such anarrangement consists of two capacitors of this type and the ohmicelectrolyte resistor between said capacitors. In the known capacitivemeasurement methods, these elements are introduced in a high-frequencyoscillating circuit in which the electrolyte resistor determines e.g.the attenuation behaviour which is evaluated subsequently. Just as inthe case of the inductive method, the expenditure for the apparatusesrequired is again very high. In view of the fact that the capacitanceswhich can be achieved are only small, the capacitive method additionallynecessitates a high measuring frequency. Furthermore, a simple linearrelationship between the conductivity and the measured quantity does notexist.

It is a major object of the present invention to provide an integrableconductivity measuring device which prevents polarization effects, whichavoids a galvanic connection to the liquid whose conductivity is to bemeasured, and which permits a simple detection of the conductivity ofsaid liquid.

This object is achieved by an integrable conductivity measuring devicefor measuring the electric conductivity of liquids, comprising

a current source device adapted to be connected to two current supplyelements through which a substantially square-wave current can be fedinto the liquid, and

a measuring circuit connected to two voltage measuring elements and usedfor determining the voltage drop in the liquid between said voltagemeasuring elements, said voltage drop depending on the electricconductivity of the liquid examined,

wherein the measuring circuit is a switch-capacitor circuit including ameasuring capacitor, a differential amplifier having a feedbackcapacitor arranged in its feedback branch, and a switch means with theaid of which the two connections of the measuring capacitor areconnected in time dependence on the behaviour of the square-wave currentto the voltage electrodes in one switching state and to the two inputsof the differential amplifier in another switching state, and

the current supply elements are implemented as current couplingcapacitors.

In short, the integrable conductivity measuring device according to thepresent application differs from the subject matter of the prior,not-prepublished PCT/DE92/00242 (WO 92/18856) corresponding to U.S. Ser.No. 08/119,243 with regard to the fact that at least the current supplyelements, which are implemented as current electrodes in accordance withPCT/DE92/00242 (WO 92/18856) corresponding to U.S. Ser. No. 08/119,243,are implemented as current coupling capacitors along the lines of thepresent invention. In accordance with an essential aspect of the presentinvention, also the voltage measuring elements are implemented asvoltage decoupling capacitors so that a complete galvanic separation isachieved between the conductivity measuring device and the liquid whoseconductivity is to be measured.

The conductivity measuring device according to the present invention isbased on a purely capacitive coupling between the conductivity measuringdevice and the liquid or electrolyte, the coupling capacitors beingpreferably integrated-technology elements, which have a thin insulatinglayer on top of a conductive layer. The counterelectrode is respectivelyformed by the liquid or electrolyte. The resultant capacitors can beproduced in a size which is sufficiently large to permit a constantcurrent to flow via two capacitors, which serve as current supplyelements, for a specific period of time. Also the resulant voltage dropin the liquid is constant for this period of time and can be detectedand further processed with the aid of two voltage measuring elements,which are implemented as voltage decoupling capacitors and which arepreferably arranged between the two current coupling capacitors.

Due to the galvanic separation, the formation of a direct-current paththrough the electrolyte or the liquid can be prevented when theconductivity measuring device according to the present invention isused. Furthermore, the application of a fixed potential to the liquid isprevented. This permits the conductivity measuring device according tothe present invention to be used in areas with high safety requirements.

The measuring circuit is adapted to be monolithically integrated on asemicondustor substrate with all components. The small size of theintegrable conductivity measuring device according to the presentinvention permits the use in connection with small sample volumes or atother points which are difficult to get at.

In the following, embodiments of the integrable conductivity measuringdevice according to PCT/DE92/00242 as well as according to the presentinvention will be explained in detail with reference to the drawingsenclosed, in which

FIG. 1 shows a circuit diagram of the conductivity measuring device;

FIG. 2 shows a time chart of currents and voltages occurring in theconductivity measuring device according to FIG. 1;

FIG. 3a shows a cross-sectional view of a coupling capacitor of theconductivity measuring device according to the present invention;

FIG. 3b shows a top view of the coupling capacitor according to FIG. 3awherein layers other than layer 32 are treated as transparent orsemi-transparent in order to show lateral placement of the layers;

FIG. 4 shows a circuit diagram of an embodiment of the conductivitymeasuring device according to the present invention;

FIG. 5a shows a top view of the arrangement of the coupling capacitorsof the conductivity measuring device according to FIG. 4;

FIG. 5b shows the electric equivalent circuit diagram of the arrangementof the coupling capacitors according to FIG. 5a;

FIG. 6a shows the effective electric equivalent circuit diagram of thearrangement of the coupling capacitors upon impression of the measuringcurrent;

FIG. 6b is a collection of waveforms depicting corresponding voltagedrops across the elements of the equivalent circuit diagram of FIG. 6a,as a function of time;

FIG. 6c is a collection of waveforms showing, as a function of time, thevoltages at points A-D, respectively, within the equivalent circuitdiagram of FIG. 6a;

FIG. 7a shows the effective electric equivalent circuit diagram of thearrangement of the coupling capacitors according to FIG. 5a uponmeasurement of the signal voltage; and

FIG. 7b shows the variations with time of the voltages across themeasuring capacitor of the conductivity measuring device according toFIG. 4.

The preferred embodiment of the integrable conductivity measuringdevice, which is shown in FIG. 1 and which is generally designated byreference numeral 1, includes a current source SQ, which is used forproducing an impressed direct current and which is adapted to beconnected to two current electrodes E1, E4 in a first polarity or in apolarity opposite to said first polarity via first to fourth switchesS1, S2, S3, S4 in dependence on the switching condition of saidswitches.

The current electrodes produce in an electrolyte EL a square-wavecurrent without any dc component.

A control device (not shown) controls the first to fourth switches S1 toS4 in such a way that the current source SQ is connected to the currentelectrodes E1, E4 alternately during a first period of time T1 in thefirst polarity and during a second period of time T2 in the secondpolarity. The first and second periods of time T1, T2 have the samelengths.

Two voltage electrodes E2, E3 are arranged between the currentelectrodes E1, E4 in the electrolyte EL, said voltage electrodesmeasuring the voltage drop V1 across the electrolyte due to theimpressed square-wave current between the current electrodes E1, E4.

The variation with time of the voltage drop V1 with reference to thefirst and second periods of time T1, T2 is shown in FIG. 2, waveformsa-c.

The voltage electrodes E2, E3 are adapted to be connected to theelectrodes of a measuring capacitor C1 via fifth to eighth switches S5,S6, S7, S8 in a first or second polarity.

The fifth, sixth, seventh and eighth switches S5 to S8 are alsocontrolled by the control device (not shown) which can be implemented asa microprocessor. The control is effected such that the voltageelectrodes E2, E3 are connected to the measuring capacitor C1 during athird period of time T3 in the first polarity and during a fourth periodof time T4 in the second polarity. As can be seen from waveforms d and eof FIG. 2 with respect to waveforms a and b of FIG. 2, the third periodof time T3 lies within the first period of time T1 and the fourth periodof time T4 lies within the second period of time T2.

Ninth and tenth switches S9, S10 lie between the two electrodes of themeasuring capacitor C1 and the inverting or non-inverting input of anoperational amplifier OPV whose output is connected to the invertedinput thereof via a feedback capacitor C2.

The control device (not shown) connects the measuring capacitor C1 tothe inputs of the operational amplifier OPV during a respective fifthperiod of time lying not within said third and fourth periods of timeT3, T4. According to the capacitance relationship between the feedbackcapacitor C2 and the measuring capacitor C1, the voltage across themeasuring capacitor V_(C1) will thus be amplified to a voltage V_(OUT)produced at the output of the operational amplifier.

In the embodiment shown, the control device (not shown) closes at theend of each fifth period of time an eleventh switch S11, which isconnected in parallel with the feedback capacitor C2, so that theswitch-capacitor circuit S5 to S11, C1, C2, OPV shown will work as anamplifier circuit. It is, however, also possible to close said eleventhswitch S11 whenever several periods of time T1, T2 have elapsed so thatthe switch-capacitor circuit will work as an integrating circuit in thiscase.

As can be seen from the curve representing the voltage drop across thevoltage electrodes E2, E3 according to waveform c of FIG. 2, the firstand second periods of time have each been selected sufficiently long forchange-over effects to decay and for the voltage V1 to assume anessentially constant value. Only after the decay of the change-overprocesses, the measuring capacitor is connected to the voltageelectrodes during the time period T3. This has the effect that chargecarriers flow across the voltage electrodes E2, E3 onto the electrodesof the measuring capacitor C1. At the beginning of this time period T3,this flow of current leads to a disturbance of the original fieldbetween the current electrodes E1, E4 and to a momentary polarization.As the charge of the measuring capacitor C1 increases, the measuringcurrent at the voltage electrodes E2, E3 tends exponentially towardszero so that the voltage electrodes E2, E3 will become current-free whenthe time period T3 is sufficiently long. In a sufficiently long thirdperiod of time T3, which depends on the the individual case, but whichcan easily be determined in an experiment, the polarization effects nolonger have any negative influence on the measuring accuracy which canbe achieved.

The conductivity measuring device is suited for an integration of theelectrodes E1 to E4, the current source circuit SQ and the amplifyingelectronic system including the switch-capacitor circuit on a singlesemiconductor substrate. By means of the monolithic integration on asemiconductor substrate, the conductivity measuring device can beminiaturized to a high degree so that measurements can be carried out insmall sample volumes or at other points which are difficult to get at,as for example in the field of invasive medical diagnostics.

The circuit components can be implemented in CMOS technology. In thiscase, the production of the electrodes can be carried out such that itis compatible with the CMOS process, since merely the additional processstep of applying a precious metal layer for the electrodes is required.

Although the conductivity measuring device is preferably suited forcomplete integration, also measuring circuits with separately arrangedelectrodes can be realized on the basis of the concept described.

The integrable conductivity measuring device according to the presentinvention differs from the conductivity measuring device which has beendescribed with reference to FIG. 1 essentially with regard to the factthat the electrodes E1 to E4 of said last-mentioned conductivitymeasuring device have been replaced by coupling capacitors CK1, CK2,CK3, CK4. As for the rest, the circuit diagram of FIG. 4 correspondsidentically to that shown in FIG. 1 so that a renewed description of thecircuit arrangement can be dispensed with. Only for the sake ofcompleteness, reference is made to the fact that the measuring capacitoris here designated by the reference sign CM, whereas the feedbackcapacitor is designated by the reference sign CR.

The coupling capacitors can be formed together with the conductivitymeasuring device such that they are integrated therein and they havepreferably the structure which will be described hereinbelow withreference to FIG. 3a, 3b.

A semiconductor substrate 30 has provided thereon a conductive layerwhich consists preferably of polysilicon 32, said conductive layer beingarranged on top of an oxide layer 31, which is a silicon oxide layer 31in cases in which a silicon substrate is used. This polysilicon layer 32forms one side of the coupling capacitor. A thin insulating layer 33consisting of silicon oxide and silicon nitride is provided on saidpolysilicon layer 32, said insulating layer 33 separating the circuitand the poly-terminal galvanically from the electrolyte 34. Thecounterelectrode of this capacitor is formed by the electrolyte 34itself, which is in direct contact with the insulating layer 33. Thesurface of the arrangement is covered by a protective oxide layer 35having an opening 37 in the area of the capacitor surface 36.

As can additionally be seen in FIG. 3b, the conductive polysilicon layer32 extends up to an extension 38 used for connection to the rest of thecircuit.

The effective width of the capacitor thus formed is the thickness of theinsulating layer 33. The resultant component acts as a capacitor,although only one of its sides exists in the conventional, solid formand consists of a material in which electrons are responsible fortransporting the current. The other side, however, is liquid, since itis formed by the electrolyte 34, the charge transport being here carriedout by dissociated ions. In view of the fact that the decisive factorwith respect to a migration of electrons within the circuit as well asof ions within the electrolyte 34 is, in the final analysis, the fieldstrength existing at the charge carrier, and in view of the fact thatthis field strength continues to be effective through the insulatinglayer 33, a displacement current can be caused to flow through thecomponent. In the course of this process, charge carriers with oppositesigns accumulate on the capacitor plates, just as in the case ofconventional capacitors. The only difference is to be seen in the factthat a negative charge of electrons will, for example, form on one side,whereas a positive charge of ions will form on the opposite side.

As is clearly shown in FIGS. 4 and 5a, the conductivity measuring deviceaccording to the present invention comprises a total number of fourcoupling capacitors C_(K1), C_(K2), C_(K3), C_(K4) of this type, whichare arranged on a common semiconductor substrate 30 in a planararrangement.

In contrast to the electrodes E1 to E4, which have been used in theembodiment of FIG. 1 and which are in direct contact with theelectrolyte via a conductive layer, a charge transfer between thecircuit and the liquid cannot take place in the present case. Incontrast to measuring cells with galvanic contact, where adirect-current path through the electrolyte exists in any case, saiddirect-current path causing possibly an electrolysis of the electrolyteif malfunction occurs, this disadvantageous effect is here avoided byusing galvanically separated cells.

FIG. 5b shows the equivalent circuit diagram of the arrangement of thecoupling capacitors according to FIG. 5a. As can be seen in said FIG.5b, ohmic divider resistors R_(EL1), R_(EL2), R_(EL3) lie between thecoupling capacitors C_(K1) to C_(K4). The CMOS switches S₅ and S₆ are(in the equivalent circuit diagram according to FIG. 7a) replaced bytheir switch-on resistance R_(ON). The switch-on resistance of theseswitches S₅, S₆ is only of importance during the pulse phase T₃ in thecourse of which the measuring capacitor C_(M) is charged.

In the reflections following hereinbelow, a square-wave current isgenerally taken as a basis, said square-wave current flowing through thearrangements from A to D or vice versa for respective identical periodsof time. As can be seen in FIG. 3, the current is produced by adirect-current source SQ whose polarity is periodically reversed withthe aid of controlled switches S1 to S4.

The process described is subdivided into two subprocesses forexplanatory reasons, viz., on the one hand, the explanation of theprocesses in connection with the impression of the measuring currentand, on the other hand, the measurement of a voltage which isproportional to the specific resistance of the electrolyte.

FIG. 6a shows the effective electric equivalent circuit diagram for theimpression of the measuring current. In order to make things easier,only the flow of current in a direction from A to D (pulse phase T₁) isexamined in the present connection, since a flow of current in theopposite direction (pulse phase T₂) will only have the effect that thesigns change. Point D is assumed to be connected to ground for theperiod of time taken into consideration. The constant current will thencause a linear rise in the voltage at the capacitor C_(K4) in accordancewith the following equation: ##EQU1##

The element of the electric equivalent circuit diagram following thefourth coupling capacitor C_(K4), viz. the third ohmic electrolyteresistor R_(EL3), causes a constant voltage drop due to the measuringcurrent in accordance with the following equation:

    V.sub.REl3 =I.sub.O R.sub.El3                              (2)

Also the other two ohmic divider resistors R_(El2), R_(El1) cause thefollowing voltage drops:

    V.sub.REl2 =I.sub.O R.sub.El2                              (3)

    V.sub.REl1 =I.sub.O R.sub.El1                              (4)

The voltage at point C is obtained in accordance with the equationfollowing hereinbelow as sum of the voltages across the fourth couplingcapacitor C_(K4) and the third electrolyte resistor R_(El3) : ##EQU2##

Accordingly, the following equation is obtained for the voltage at pointB: ##EQU3##

The upper capacitor C_(K1) has the same size and the same structuraldesign as the capacitor C_(K4) and causes a voltage drop whichcorresponds to that caused by said capacitor C_(K4) and which satisfiesthe following equation: ##EQU4##

With regard to the further evaluation, it is now of decisive importancethat, although the voltages at points B and C are absolutely rampshaped,the difference between them is constant and depends only on themeasuring current and the electrolyte resistance. Hence, the followingequation holds true:

    V.sub.B -V.sub.C =I.sub.O R.sub.El2

For detecting this differential voltage, two additional couplingcapacitors C_(K2) and C_(K4) are used, which are arranged between theabove described capacitors C_(K1) and C_(K4). During a time T₃ after thechange of polarity of the measuring current, i.e. when stationaryconditions occurred with respect to the ohmic voltage drop across theelectrolyte, these coupling capacitors C_(K2) and C_(K4) are connectedto the measuring capacitor C_(M) via the switches S₅ and S₆. Ananalogous course of action is taken when the current flows in theopposite direction. In this case, the switches S₇ and S₈ are used in acorresponding manner during the period of time T₄ within the period oftime T₂.

For analyzing the process, the ohmic voltage drop across the dividerelectrolyte resistor R_(El2) is shown in FIG. 4 as a voltage source.This is admissible, although the original distribution of current isinterfered with due to the fact that the branch including the measuringcapacitor C_(M) is charged, since this branch becomes current-free againafter a short period of time so that, afterwards, the conditions will bethe same as if no measuring capacitor C_(M) were provided.

The following boundary conditions apply to the charging process of C_(M):

the charging takes place exponentially, since only resistors andcapacitors are located in the circuit in question.

The time constant is defined by the dual switch-on resistance of theCMOS switches as well as by the series connection of the couplingcapacitors C_(K2), C_(K3) and the measuring capacitor C_(M). C_(K2)corresponds to C_(K3) in this case. The charging time constant can beexpressed by the following equation: ##EQU5##

The time constant t_(M) obtained with the realized circuit lies withinthe range of a few 10 ns so that charging of the measuring capacitorC_(M) within a very short time, in comparison with the duration of thevoltage ramp of some 10 Is, is guaranteed. The final value of thevoltage across the measuring capacitor C_(M) is obtained from thecapacitive division ratio of the existing capacitors in accordance withthe following equation: ##EQU6##

Having obtained this voltage V_(CMEnd), a measuring signal which isproportional to the electrolyte resistance R_(El2) is now available. Dueto the given geometrical conditions of the measuring arrangement, thesecond electrolyte divider resistor R_(El2) is linked via a constantwith the specific resistance and by the formation of a reciprocal withthe conductivity of the electrolyte. This constant can quasi be regardedas a "cell constant" of the arrangement.

The variations with time of the voltage across the measuring capacitorC_(M) are shown in FIG. 7b.

When the measuring signal has been transmitted to the measuringcapacitor C_(M), the switches S₅ and S₆ are reopened, the charge on themeasuring capacitor C_(M) being maintained in the course of this process(cf. FIG. 7a). For the purpose of further processing, said charge isintroduced into the switch-capacitor circuit according to FIG. 3 bymeans of additional switches S₉ and S₁₀ and then amplified. In thecourse of this process, the charge stored on the measuring capacitorC_(M) is transferred to the feedback capacitor C_(R), the capacitanceratio of C_(M) to C_(R) defining the amplification factor. The outputvoltage V_(OUT) is available as an output value after each chargingprocess. If the operational amplifier is connected as an integrator inthe case of which the voltage across the feedback capacitor C_(R) isonly reset after several cycles by the eleventh switch S₁₁, it will bepossible to amplify the output signal still further.

The conductivity measuring device according to the present inventionexcludes polarization effects, it prevents electrolysis processes in theelectrolyte, and it also excludes or markedly reduces drift errorscaused by an absorption of ions or by contamination in the area of themeasuring elements. The electrolyte is not at a fixed potential, wherebythe range of application of the conductivity measuring device accordingto the present invention is enlarged still further. In view of the factthe electrolyte need not be in contact with metals, undesired chemicalreactions, such as catalytic reactions, are avoided.

We claim:
 1. An integrable conductivity measuring device for measuringthe electric conductivity of liquids, comprisinga current source deviceadapted to be connected to two current supply elements through which asubstantially square-wave current can be fed into the liquid, and ameasuring circuit connected to two voltage measuring elements and usedfor determining the voltage drop in the liquid between said voltagemeasuring elements, said voltage drop depending on the electricconductivity of the liquid examined, said voltage measuring elementscomprising voltage decoupling capacitors, wherein the measuring circuitis a switch-capacitor circuit including a measuring capacitor, adifferential amplifier having a feedback capacitor arranged in itsfeedback branch, and a switch means with the aid of which the twoconnections of the measuring capacitor are connected in time dependenceon the behaviour of the square-wave current to the voltage electrodes inone switching state and to the two inputs of the differential amplifierin another switching state, and the current supply elements areimplemented as current coupling capacitors.
 2. An integrableconductivity measuring device according to claim 1, wherein eachcoupling and decoupling capacitor is formed as a conductive layer on aninsulating intermediate layer, which, in turn, is positioned on asemiconductor substrate, and is covered by an insulating surface layer.3. An integrable conductivity measuring device according to claim 2,wherein the insulating surface layer consists of silicon oxide andsilicon nitride,the conductive layer consists of polysilicon, and theinsulating intermediate layer consists of silicon oxide.
 4. Anintegrable conductivity measuring device according to claim 1, whereinthe current source device comprises a direct-current source which isadapted to be connected via first, second, third and fourth switches tothe current coupling capacitors in a first and in a second polarity. 5.An integrable conductivity measuring device according to claim 1,comprising a control device, which controls the first, second, third andfourth switches in such a way that they connect the current source tothe current coupling capacitors alternately during a first period oftime in the first polarity and during a second period of time in thesecond polarity.
 6. An integrable conductivity measuring deviceaccording to claim 5, wherein the switch means has fifth, sixth, seventhand eighth switches,the control device controls the fith to eighthswitches in such a way that they connect the voltage decouplingcapacitors to the measuring capacitor during a third period of time in afirst polarity and during a fourth period of time in a second polarity,and the third period of time lies within the first period of time andthe fourth period of time within the second period of time.
 7. Anintegrable conductivity measuring device according to claim 5, whereinthe first and second periods of time are of equal length so that thesquare-wave current has no dc component averaged over time.
 8. Anintegrable conductivity measuring device according to claim 1, whereinthe measuring circuit is provided with an amplifier circuit which isadapted to be connected to the measuring capacitor via ninth and tenthswitches.
 9. An integrable conductivity measuring device according toclaim 2, wherein the control device controls said ninth and tenthswitches in such a way that they connect the amplifier circuit to themeasuring capacitor during a fifth period of time, which does not liewithin said third and fourth periods of time.
 10. An integrableconductivity measuring device according to claim 8, wherein themeasuring circuit is provided with feedback capacitor located in thefeedback branch of the amplifier circuit.
 11. An integrable conductivitymeasuring device according to claim 10, comprising an eleventh switchconnected in parallel to the feedback capacitor,wherein the controldevice controls the eleventh switch in such a way that the feedbackcapacitor is discharged after each fifth period of time.
 12. Anintegrable conductivity measuring device according to claim 10,comprising an eleventh switch connected in parallel to the feedbackcapacitor,wherein the control device controls the eleventh switch insuch a way that the feedback capacitor is discharged whenever aplurality of periods has elapsed.